As of today we marked it as stable. We support the latest NixOS release and unstable.
What is the ambition on the long term? Would you be open to people sending you PRs for baremetal servers profiles? I have been using it for my infrastructure with some changes and it’s nice. 
Long-term we want to extend the provider list here more: https://numtide.github.io/srvos/nixos/hardware/
and have more tutorials how you can get started on these platforms. Otherwise we just add stuff we feel we copy too much around.
In the past I have added some bare-metal servers here: https://github.com/NixOS/nixos-hardware/tree/master/supermicro but than stopped because they didn’t really needed a lot of configuration or they are too configurable unlike laptops. So the model number does not really give away what’s inside.
What would be cool is is some generic baremetal configuration that works with most servers out-of-the-box with little configuration needed (Most of the time it’s just Intel vs. AMD and than some Sata/NVME drivers). Than we don’t need to run a nixos-generate-config step every time.